Color displays using raster-scanned cathode ray tube technology rely upon accurately controlling the position and intensity of an image beam scanned in a raster pattern to produce a displayed color image. Future high resolution CRT displays, including those for high definition television, will require even more performance in controlling image beam deflection and/or intensity to present an accurate color image.
A color CRT tube includes a phosphor-coated display screen and an electron beam generator. The phosphor coating on the display defines an array of picture elements (pixels), with each pixel including three color phosphor elements--red, green and blue (RGB). CRT systems use electron beam generators with either one or three image beams. In single-image-beam systems, the color image is a composite produced by precisely controlling the position of the image beam within each pixel relative to the three color elements (providing a composite color output corresponding to the color component ratio) as the beam is raster-scanned over the pixel array. In three-image-beam systems, the color image is a superposition of three color component images produced by separately controlling the intensities of each beam as they are scanned across respective color elements of a pixel.
A significant cost factor in producing high resolution color picture tubes is the need to provide a highly reliable and uniform phosphor screen coating with phosphor color coating elements that do not degrade in color, profile or response time during the anticipated life of the display device. In addition, the accuracy and reliability of the electron beam focusing and positioning components is important, and is critical in single-image-beam systems where slight variations in beam position within a pixel can cause color distortions.
Several mechanisms cause deterioration in color image quality over the life of a color CRT system. Phosphors inevitably lose efficiency in converting electron impact energy into optical radiation. Nonuniformities in phosphor thickness and chemical quality also show up as variations in image intensity and color accuracy. Many CRT applications require that some part of the screen receive much more electron beam excitation than others (such as closed captions and software menus), so that phosphor degradation can occur at different rates over the screen. Different color phosphors will degrade over time at different rates, with blue phosphor tending to have a longer life than red phosphor. In addition, electronic component degradation and mechanical misalignments, as well as locating the CRT display next to other electronic equipment, can cause unpredictable distortions in the magnetic or electronic fields used to control beam deflection.
Many of these mechanisms also adversely affect the image quality of monochrome displays. In addition, other display technologies--such as LCD and LED--experience image quality problems. In particular, while LCD and LED systems do not require any beam positioning as in CRT systems, they do experience significant problems in controlling intensity at the pixel level. For LED displays, diodes with uniform light output (intensity) are difficult to produce in quantity, so that improving "pixel" image quality requires more stringent component selection criteria that increase expense. For LCD displays, variations in capacitive field strength between row/line conductors cause variations in light output (intensity) for a "pixel", and therefore, adversely affect image quality.
Accordingly, a need exists for a system for correcting display image distortions at the pixel level, whether caused by errors in intensity or other factors such as positioning an image beam, based on detected image errors in the output display image.